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Tuesday, 8 April 2014

"Those that sing together stay together": Migration and the evolution of duetting in songbirds. Proceedings of the Royal Society B, March 2014

Migration and the evolution of duetting in songbirds.

David M. Logue, 1
Michelle L. Hall, 2

1 - Departamento de Biología, Universidad de Puerto Rico, PO Box 9000, Mayagüez PR 00681-9000, Puerto Rico
2 - Department of Zoology, University of Melbourne, Melbourne, Victoria 3010, Australia

Published 11 March 2014 doi: 10.1098/rspb.2014.0103

Proc. R. Soc. B 7 May 2014 vol. 281 no. 1782 20140103

Many groups of animals defend shared resources with coordinated signals. The best-studied of these signals are the vocal duets produced by mated pairs of birds. Duets are believed to be more common among tropical-breeding species, but a comprehensive test of this hypothesis is lacking, and the mechanisms that generate latitudinal patterns in duetting are not known. We used a stratified sample of 372 songbird species to conduct the first broad-scale, phylogenetically explicit analysis of duet evolution. We found that duetting evolves in association with the absence of migration, but not with sexual monochromatism or tropical breeding. We conclude that the evolution of migration exerts a major influence on the evolution of duetting. The perceived association between tropical breeding and duetting may be a by-product of the migration–duetting relationship. Migration reduces the average duration of partnerships, potentially reducing the benefits of cooperative behaviour, including duetting. Ultimately, the evolution of coordinated resource-defence signals in songbirds may be driven by ecological conditions that favour sedentary lifestyles and social stability.

Coordinated signals are used to defend shared resources in diverse taxa ranging from social shrimp Synalpheus to lions Panthera leo. The vocal duets and choruses produced by songbirds are particularly striking for the remarkable precision with which individuals coordinate their vocalizations. Researchers have investigated the function of avian duetting, sexual dimorphism of the brain regions associated with song and the neural basis of coordinated singing. However, factors driving the evolution of duetting are poorly understood. We investigated the evolution of duetting to advance our understanding of cooperative group signalling and integrate that line of research with the rich literature on the evolution of cooperation.

Intensive experimental research has shown that avian duets function primarily in cooperative defence of territories and mates. Consistent with this functional interpretation, there are positive associations between duetting and both pair-bond duration and year-round territoriality in North American birds. Avian duets are thought to be more common in the tropics, and may be associated with sexual monochromatism. Conclusions about the evolution of duetting based on existing comparative analyses are limited by the scopes and methodologies of those studies. Early comparative studies did not take phylogeny into account. More recent studies included only partial control for phylogenetic non-independence, and were restricted in geographical and phylogenetic scope.

We hypothesize that seasonal migration may be an important factor driving the evolution of cooperative duetting. Theoretical and empirical work suggest that long-lasting relationships favour the evolution of cooperative behaviour. Factors that facilitate long-term relationships are therefore predicted to promote the evolution and maintenance of cooperation, whereas those that disrupt relationships are likely to drive the evolutionary loss of cooperation. The cooperative functions of duetting may be of limited benefit in migratory songbirds, which have shorter pair bonds owing to higher rates of divorce between breeding seasons. Migration also has the potential to explain other previously identified correlates of duetting, since migration covaries with latitude and is associated with sexual dichromatism. Our hypothesis predicts a negative correlation between the evolution of duetting and migration that is stronger than correlations between duetting and either breeding latitude or sexual monochromatism.

We investigate the evolution of duetting with a broad-scale comparative analysis. We include both duets and choruses in our analysis (using the term ‘duets’ inclusively, for convenience) because the two are related both structurally (vocalization by one individual prompts a coordinated vocal response by one or more other individuals) and functionally (both function in cooperative resource defence). We sample across the entire songbird phylogeny (suborder: Passeri), and test for correlated evolution between duetting and migration, breeding latitude and sexual monochromatism. Our findings challenge long-standing hypotheses about the evolution of duetting, and reveal a previously untested factor affecting the evolution of this cooperative behaviour.

Figure 2
The relationship between duetting, migration and breeding latitude in a sample of 372 songbirds. The proportion of species that duet (black lines) and migrate (grey lines) are plotted as a function of the latitudinal centre of their breeding range. Tropical latitudes are shaded. Numbers in parentheses indicate the number of species that contribute to the duetting and migrating data, respectively. Sample sizes differ because species with a mixed migratory strategy were excluded from the estimates of migration frequency. The two most extreme bins were collapsed because they included few cases.

Our data suggest that the evolution of migration is a major driver of the evolution of duetting in songbirds. We hypothesize that migration reduces the average duration of partnerships, thereby reducing the likelihood that cooperative behaviours such as duetting will evolve and be maintained. Given that ecological variation drives the evolution of migration, ecological stability may indirectly influence the evolution of duetting and other cooperative behaviours between mated songbirds by promoting year-round residency (i.e. the absence of migration). Our findings help to link duetting research to the broader context of research on cooperative behaviour, and establish a tentative link between ecological stability and the evolution of cooperation in songbirds.

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